It is known that the condition of a bank of serially connected battery cells is dependent upon the condition of the individual cells within the bank. In order to detect a deteriorating one of the cells the condition of a given cell must be measured individually or as part of a plurality of different groups of cells. In particular, to monitor battery voltages, it has been common to use a differential multiplex method in which flying capacitors or high voltage multiplexes are used to collect voltage from different cells. In either case, at least one wire per measured cell has been used to provide a sensing voltage for the multiplexing circuitry. The disadvantage of such a system lies mainly in the number of sensing wires.
U.S. Pat. No. 5,099,211 permits measurements of multi-cell arrangements with a single sensing wire. According to this patent one lead of a single voltage response switch is connected to each of the interconnecting battery terminals and a common terminal. In addition, the switch is connected between this common terminal and one of the end terminals of the bank of batteries. A ramp voltage is then applied between the other end terminal and the common terminal, the ramp voltage being such as to extend from zero to a voltage in excess of the total voltage of the bank of batteries. As the voltage is ramped up each switch is opened then closed, in succession. This arrangement gives rise to current pulses and both ramp voltage and instantaneous current through this arrangement are coordinately provided as output. The voltage between discrete pairs of pulses are an indication of the cell voltage.
A serious drawback of the invention disclosed in U.S. Pat. No. 5,099,211 is that it is difficult to accurately measure voltages below a few volts due to the nature of the switching. The method is also noise sensitive and might have problems in proper operation in the presence of a ripple voltage. Furthermore, it employs a rather complicated switching circuit which has to be built into every battery sensing wire connector.
It is an object of the present invention to provide a multi-cell battery monitoring system, method and apparatus utilizing a single sensing wire which is capable of accurately measuring voltage, including voltages below one volts, in a manner which is resistant to noise and the effects of a ripple voltage.
Further and other objects of this invention will become apparent to a man skilled in the art when considering the following summary of the invention and the more detailed description of the preferred embodiments illustrated herein.